Hearing aid mechanisms that are largely impervious to the leakage of sound energy at audio frequencies



July 22, 1969 J. w. HAGGERTY 3,457,375

HEARING AID MEGHANISMS THAT ARE LARGELY IMPERVIOUS To THE LEAKAGE OFSOUND ENERGY AT AUDIO FREQUENCIES Filed Oct. 1. 1965 5 Sheets-Sheet 1/lVl/ENTOR .JOHN W. HAGGERTY ATTORNEYS.

July 22, 1969 J. w. HAGGERTY HEARING AID MECHANISMS THAT ARE LARGELYIMPERVIOUS TO THE LEAKAGE OF SOUND ENERGY AT AUDIO FREQUENCIES Filed00T.. 1, 1965 5 Sheets-,Sheet 2 FIG. 8

FIG. IO

RADIAL VIBRATION (RMS. CM.)

O O O m FREQUENCY (CRS) 5y PMAM A TOR/VH GERTY 3,457,375 E LARG YIMPERVIOUS TO THE AT AU FREQUENC J. W. HAG NISMS T AR SOUND E RGY July22, 1969 HEARING AID MECHA EAKAGE OF Filed 001'.. l, 5

IES 5 Sheets-Sheet 5 FREQUENCY FIG. l2

AMPLIFICATION BY HEARING AID WITH NO ACOUSTlC FEEDBACK INSTABILITYREGION AMPLIFICATION BY HEARING AID WITH AcousTlc F|G. x4

/Nl/ENTOR JOHN W. HAGGERTY @y fm/Lm M ArmR/vys FREQUENCY FEEDBACK AT ONEFREQUENCY nted States Patent O 3,457,375 HEARING AID MECHANISMS THAT ARELARGELY IMPERVIOUS TO THE LEAK- AGE OF SOUND ENERGY AT AUDIO FREQUENCIES.lohn W. Haggerty, 1814 Buckingham Road, Los Angeles, Calif. 90019 FiledOct. 1, 1965, Ser. No. 491,958 Int. Cl. H041 25/02 U.S. Cl. 179-107 16Claims ABSTRACT OF THE DISCLOSURE This invention relates to improvedhearing aid systems, and relates particularly to improved housings forhearing aid ear receivers and their connection to earmolds.

The hearing aid industry trend toward amplifiers having higher reservepower potential has not affected the basic limitation of acousticalfeedback on hearing aid system amplification stability andbandwidth-amplitude product. Unless this limitation is modified, thepercentage of deaf persons who can benefit from hearing aids will notincrease. It has long been recognized that with increasing amplificationthere is a growing need for control of hearing aid feedback howl andwhistling which both annoy the wearer of a hearing aid system andprevent him from employing his hearing aid to its full amplificationpower capability. This recognition is so universal that many powerfulhearing aid systems are sold with custom fitted earmolds whose primaryfunction is to seal against leakages contributing to acoustical feedbackat the same time as minimizing the air volume between the eardrum andthe transducer. Custom fitted earmolds do not, however, eliminate allacoustical feedback. In fact it is not at all uncommon for a person withnormal hearing when conversing with a wearer of a properly fittedhearing aid to hear small portions of his speech being rebroadcast as aresult of the acoustic leakage from the hearing aid system, even thoughthe system is not howling or whistling. As hearing aid systems arerequired to deliver higher and higher amplification for theincreasinglydeaf, they almost invariably oscillate (become unstable) atsome audio frequency. For such persons, hearing 'aid benefits arelimited to a delicate amplification setting at which feedbackoscillation is a continuous incipient tendency. Hearing aid operation atthis setting thus has a ringing characteristic which stops and startswith various head angles and positions. The frequency of ringing andhowling depends on and varies with the hearing aid equipment in use, aswell as its placement on the wearer.

To reduce loss of human potential among the profoundly deaf there hasbeen active mutual interest and cooperation among hearing aidmanufacturers, dealers, and educators of the deaf. Powerful body-wornconventional style hearing aids are being employed at the limits oftheir amplification capa-bility in increasingly successful speechtraining of the nearly totally deaf. But this speech training becomesmore difficult and less satisfactory when the hearing loss of a traineeexceeds 60 or 70 decibels. For persons with over 90 decibels hearingloss, speech 3,457,375 Patented July 22, 1969 ice training time isexceedingly long and the resulting speech quality is far from normal.

Delivery of large am-ounts of acoustical :amplification for speechtraining purposes with present hearing aid equipment now requires largeearmuffs to be worn over ear receivers. These large sound-containmentdevices confiict with the cosmetic needs of the handicapped deaf.Understandably the higher amplification enabled by the large earmutfs islimited to the classroom and some home situations.

As is well known, the speech of persons deaf from birth is an accurateimitation of what these handicapped persons perceive through theirdiminished sense of hearing. The lower the residual hearing capacity ofa person, the more the hearing aid is relied upon. That the best ofpowerful hearing aid systems are not equal to the task of amplifying forpersons with 80 decibels or more hearing loss is demonstrated by theindistinct speech of these persons as mentioned above. The soundsmouthed by this class of handicapped persons create a profund socialbarrier which causes predictable emotional and economic damage. Theprocess by which hearing aids deliver incresingly unintelligible soundsto the increasingly deaf is explained in this specification.

An object of this invention is to provide a practical ear receiverhousing and earmold connection construction which effectively containsacoustical energy within hearing aid ear receivers over the completeaudible frequency range for the highest sound pressure levelsdeliverable by hearing aid amplifiers coupled with ear receivertransducers.

A further object is an improved hearing aid system capable of providinggreater acoustical amplification over a greater audio frequency rangethan now possible in the arts of cosmetically acceptable hearing aids.

Another object is the provision of adaptation devices for existingconventional body-worn hearing aid systems which travels through soundinsulation shall herein be termed leakage. In one form, the inventioncomprises a :lightweight housing structure which is largely imperviousto the passage of sound energy at audio frequencies. The unavoidablestructural resonances of this stiff housings concentric shells aremutually de-tuned and individually damped to reduce those acousticenergy leakages which are greatest at those structural resonances. Thisstructure provides improved hearing aid ear receiver devices, orsignificant portions of this structure may be incorporated by additiononto hearing aid ear receiver housings of existing design. Furtherfeatures of devices in accordance with the invention are that themechanical connection of the ear receiver to the earmold as well as theelectrical connection of the ear receiver to the amplifier are made moresecure against the passage of sound energy to the air. Each decibel ofacoustic energy leakage reduction enables another decibel of wide-bandamplification by hearing aids, and the present invention enables over l5decibels improvement beyond present hearing aid amplification stabilitylimits.

` In accordance with the invention, the following features are employedin ear receivers and ear receiver housings, singly or in combination:

(A) The housing portions are made stiff (over 500,000

p.s.i./inch radial stiffness calculated as a pressure vessel) andpreferably constructed of materials whose bulk specific acousticimpedance exceeds 7,000,000 kg./sec ondxmeterz. Three-dimensionalcurvatures as on a sphere are useful to increase wall radial stiffnesswith minimum weight penalty.

(B) The housing portions are made gas-leak-tight to prevent airborneacoustic energy waves from bypassing the mechanical containmentbarriers. Rolled compression joints are avoided and housing joints aremore effectively sealed by other permanent means.

(C) The walls of the housing portions are preferably damped as much aspossible to reduce their inevitable mechanical resonance effects in theaudio frequency range. It is desirable to reduce wall thickness when incontact with damping materials, to facilitate transmission of anyvibration energy into the damping materials by mechanical transformeraction.

(D) The housing portions are alternatively made of multiple-wallconstruction. Preferably, each such wall has a fundamental mechanicalresonance frequency differing by a substantial amount, and at least byl/s octave, from that of any other wall. Coupling between these resonantelements is minimized by individual acoustical damping of each wallwhich tends to vibrate and by providing the greatest acousticalimpedance mismatch at their interfaces. This mismatch is made usefullylarge by an airspace between the multiple walls.

(E) The physical size of all housing surfaces exposed to the externalair is minimized. This acoustic radiation surface is significantlyreduced by sealing leakage from the front surface of ear receivers atthe periphery of the housing abutment with the earmold.

Suitable physical arrangements embodying the above constructionpractices are shown in FIGS. 1 through 7 of the accompanying drawings inwhich:

FIG. l is a perspective, partly cut away view of an ear receiver housingincorporating a preferred exemplification of the invention;

FIG. 2 is a sectional view of the housing drawn in FIG. 1;

FIG. 3 is an exposed view of an electrical connector assemblyconstructed in accordance with the invention;

FIG. 4 is a sectional view similar to FIG. 2, showing connection of thepreferred ear receiver housing to an adjoining earmold which isspecially configured in cross section;

FIG. 5 is a view partly in section of a modification unit in accordancewith the invention for a conventional hearing aid ear receiver whoseconstruction includes a plastic back housing shell;

FIG. 6 is a view partly in section showing a different modification unitcoupled to the hearing aid ear receiver of FIG. 5;

FIG. 7 is a view partly in section showing a modification to anothercommercially available hearing aid ear receiver whose constructionincludes a metal back housing shell;

FIG. 8 is an acoustical energy ow diagram useful in explaining theinvention and representing a hearing aid system as an amplifyingservomechanism;

FIG. 9 is a simplified sectional View through the auditory canal of ahuman left ear when fitted with earmold and ear receiver, lookingupwards, and is useful in understanding many of the major acousticenergy leakages in hearing aids and their total contribution toacoustical feedback to the hearing aid microphone;

FIG. 10 is a log-log plot of sound pressure level on the surface ofvariously constructed one-inch diameter hemispheres as a function oftheir radial vibration amplitude as driven by a uniform internal soundpressure amplitude whose frequency is adjusted over the specified range;

FIG. 11 is a three dimensional plot of the response surface of anidealized hearing aid system which is free of any acoustical feedback,the scale units being arbitrary with amplification limited to that levelestablished in FIG. 13;

FIG. 12 is a two dimensional plot of a section through the responsesurface of FIG. l1 at the position indicated by dashed lines;

FIG. 13 like FIG. 11 is a three dimensional plot of the response surfaceof an idealized hearing aid system, but which system includes acousticfeedback at one frequency only; and whose amplification is limited tothat level above which the hearing aid oscillates;

FIG. 14 is a two dimensional plot of a section through the responsesurface of FIG. 13 at the position indicated by dashed lines.

An example of a hearing aid device in accordane with the invention isshown in FIGS. 1 through 4. These figures are not drawn to scale. Thedevice is an exterior housing 18 with electrical socket connector 30which together with an electroacoustic transducer 28 comprise an earreceiver worn by a person as part of a conventional body-worn stylehearing aid. The ear receiver is normally connected into a custom fittedearmold 42 (FIG. 4 only) by a standard snap-fitting consisting of ahollow plug shape 21 and surrounding garter spring 48 seating in astandard socket fitting 46 in the earmold 42. The skin-side surface ofthe earmold 42 is molded to be closely complementary to the centralconvoluted manifolds of the wearers outer ear and ear canal, while thesnap-fitting side of the earmold 42 is substantially fiat. The standardhollow plug 21 is on the central axis of the ear receiver, and alsodefines a substantially coaxial hole which channels the full soundintensity from the transducer 28 to the eardrum by means of a matchingsound port in the earmold 42.

The ear receiver is also connected in conventional fashion to a portablemicrophone and audio amplifier (not shown) by means of an electricalcord whose plug end 38 only is shown. Because the portable audioamplifier is usually worn on the chest, the electrical cord spanning thedistance between ear and chest is usually no longer than two feet. Theear receiver assembly functions to transform amplified electrical audioenergy into acoustical energy immediately adjacent to the earmold. Earreceiver sizes are a compromise between cosmetic demands and the demandsof generating high acoustic intensities, and ear receivers fromcommercial hearing aid manufacturers are now fairly uniform in size.

The housing 18 for the ear receiver must (for reasons to be relatedlater) contain sounds of all audio frequencies and magnitudes within thelimits of the housing and within the connection of' said housing 18 tothe adjoining earmold 42. To this end wall stiffness, resonances andleakage paths are, individually or in combination, selected or minimizedin accordance with consideration now to be given.

There is no limit to the degree of sound containment and insulationdesirable in a hearing aid ear receiver assembly with an earmold.However, referring to FIG. 9, the upper limit to the practical degree ofsound insulation is set by the leakage path from the inner ear ateardrum 96 via skull bones and flesh to the air. Measurements by vonBekesy (published in 1960) of sound leakage by the path 80 indicate thatabout 40 decibels (fairly uniform with frequency) of sound insulation isthe magnitude of this upper practical Sound insulation limit.

It is well known and easily calculated that in series with any soundleaking from the ear receiver and earmold assembly is about 30 decibelsof sound insulation caused by spatial separation 94 of these leakagesfrom the hearing aid microphone 95. This spatial insulation effect islargely independent of frequency.

Measurements by the inventor of sound leakages from most types ofcommercially available conventional ear receiver disclosed that thesound insulation values drop invariably to only 10 decibels or less atsome frequency dependent upon the particular ear receiver housing.Measurements of insulation effectiveness against leakage through theseear receiver types `was made by cylically plugging and unplugging thesound output port 21 of the ear receiver which was energized at manyvarious steady frequencies. The decibel difference (indicated on a soundintensity measurement system) between the plugged and unpluggedcondition of the ear receiver was the inventors measure of soundinsulation by the ear receiver housing at a particular frequency. Aswill be explained later, the worst insulation frequency profoundly andadversely affects the hearing aid system behavior. The design ofhousings for ear receivers becomes one of the most critical elements ofhigh-power hearing aid system operation, capable (as is now prevalent inthe industry) of largely incapacitating the usually high qualityelectronic com-ponents of such systems by high frequency instability orhowl.

Still referring to FIG. 9, the leakage paths through the said earreceiver types can be identified as rear housing Shell resonance exure86; electrical socket hairline crack air passages 88; rolled compressionjoint hairline crack air passages 90; and front housing resonanceflexure 92. The mechanical resonance flexure leakages are fairly sharplytuned (very dependent on excitation frequency). Because properly fittedearmolds 42 seal leakage 82 from the ear canal and leakage 84 from thesnap fitting far better than ten decibels, the ear receiver housingleakages listed above comprise the present largest sources of acousticfeedback in hearing aids.

I have explained that the insulation performance of ear receiverhousings can theoretically and usefully be improved from the presentsub-l0 decibel level to the 40 decibel limit imposed by skull leakage 80or the lesser limit imposed by earmold leakage 82. Later I shall discusswhy this improvement is a necessary and valid objective. As statedbefore, the present invention enables over 15 decibels improvement inacoustic energy leakage reduction, to a minimum insulation level about25 decibels from the present l0 decibels, still beneath the theoreticalobjective of insulation of 40 decibels, but still within the limits ofcosmetically acceptable sizes and weights. By extension of the designdirections indicated in this specification, it is possible to achievethe 40 decibels acoustic insulation objective, but such assemblies arethought by the inventor to be excessively heavy and large; although manypersons may not be able to hear satisfactorily without such large andheavy devices. Furthermore at about 25 to 30 decibels insulation, thefit of an earmold becomes more critical than now required in the hearingaid industry; earmold leakage path 82 must be as free of leakage as isthe ear receiver.

To explain partially how the present invention achieves its high degreeof sound insulation at small size and weight penalties, refer to FIG.l0. This graph of vibration, frequency, and sound pressure level (SPL)applied to one-inch diameter hemispheres vibrating radially. Soundpressure level (SPL) is measured at the outer surface of the vibratinghemispherical shells, with SPL (decibel) scale referred to somearbitrary reference pressure level. Curve P applies to a plastichemispherical shell with damping .01 of critical; and curve DA appliesto a similarly dimensioned aluminum shell with damping .05 of critical.Each shell is vibrating as forced by the same uniform SPL inside theshell. The difference of resonance frequencies is primarily the resultof the differences of density and stiffness of the respective materials.In FIG. is shown the combined effect of a material substitution anddamping differences. Notice that at the low frequencies (below theresonance peaks) the wall vibration amplitude is essentially that samebulge amplitude resulting from a static internal pressure. At resonance,curve P peaks at 75 decibels SPL while curve DA peaks at 47 decibelsSPL, a difference in this example of 28 db of which 23 db is assignableto stiffness difference and 5 db assignable to the damping increase andits dissipation of resonance magnification effects. Because .01 is arather high value of internal hysteresis damping for homogeneousthermosetting plastic, and because .05 is a conservative amount ofdamping obtainable from available techniques, then the calculated 28decibels improvement in this example is conservative. It is apparentfrom this example that the greatest acoustic leakage through suchhemispheres occurs at their mechanical resonances; and so it tends to bewith ear receiver housing walls. FIG. 10` also illustrates thatstiffness is an immediate way to achieve sound containment; and if itwere possible to avoid resonances, another 20 decibels insulation wouldbe possible.

Of course the curves of FIG. 10 are simplified representations (showingonly a single resonance) of real nonspherical housing wall vibrationsand the contribution of these vibrations to acoustic feedback in hearingaid systems. But overtone resonances are usually of smaller magnitudethan fundamental resonances and are usually easily damped with the samemeans as the fundamental resonances are damped.

Note also that by limiting all amplified frequencies to those below thefundamental vibratory resonance of the ear receiver housing, it ispossible to obtain lvastly higher sound insulation values from even theplastic material. This enables (as will be shown later) vastly higheramplication levels within the limited frequency band -by a hearing aid.Powerful aids whose frequency is limited to under about 500 cycles persecond are known, but these are essentially sound magnitude indicatordevices not speech transmission devices. The limited frequency responseamplification so achieved does not satisfy the speech amplificationrequirements of the profoundly deaf whose needs quite often increase orat least remain constant up to over 8000 cycles per Second.

These considerations enable better appreciation of the unique elementsand relationships of combinations in accordance with the invention asshown in FIGS. 1 through 4.

The transducer 28 generates Waves of acoustical pressure within thehousing structure made of elements 20, 22, 24, and 26. Housing frontcase 20 is of a cylindrical shape with a circular disc spanning itsinternal diameter at about its mid-length. It is made of material whosespecific acoustic impedance exceeds 7,000,000 kg./second meter2, in thiscase aluminum suitable for anodizing. Centrally located on the circulardisc and coaxial with the cylindrical case 23 is the hollow-sound-portand earmold-snap-fitting 21. On the opposite side of the circular discis `a circumferential positioning ledge which spaces transducer 28slightly away from the disc surface. The Volume defined between the discand the transducer is employed for transducer diaphragm vibratorymotions and resonant chamber tuning in the standard manner. It isdifficult to construct a lightweight fiat disc element in 20 which isover 500,000 p.s.i. per inch bulge stiffness; relatively largethicknesses or heavy materials are required. Instead of such expedients,vibratory transmission leakage 92 by the flat disc element of 20 iscontained by the unique structure described immediately below.

Referring again to FIG. 9, acoustical leakage 84 from the snap connectorjoint as well as mechanical resonance leakage 92 from the housing frontcase disc are reduced by a damping pad 40 (see FIG. 4) disposed in acontinuous generally circular pattern surrounding the snap connector 21.The exterior section 23 of the housing front case cylinder forms a skirtextending from the perimeter of the housing.

When the housing is fitted to an earmold as seen in FIG. 4, this skirtextends into and nearly fills a circular groove 41 in earmold 42 whilethe damping pad 40 abuts the flat surface of the earmold 42. Aperipheral labyrinth sealk is completed between the earmold and the earreceiver by caulking material 44 filling the clearance The rearwardextending cylindrical element of the i housing front case surrounds theperiphery of the transducer 23 in a snug fit. Extending further rearwardis a skirt which serves to locate the housing rear walls 22 and 26. Thethickness of this locating skirt tapers from about .030 inch at its baseto about .008 at its end. This taper is complementary to the thinning ofperipheral edge sections of the rear housing walls 22 and 26. Thetapering and thinning of the wall sections functions as a mechanicaltransformer for housing wall vibrations tending to couple the Wallvibrations more effectively into damping materials 24 locatedinterstitially between the tapers of 20 and 22 and between 20 and 26.Thixotropic viscous compounds or mixtures as well as solids and rubberswith high internal mechanical hysteresis can serve as the dampingmaterials 24. Because it is convenient in this particular design tocombine air leakage sealing and structural joining with dampingfunctions, the damping element 24 can be a thin rubber bond cured inplace with small proportions of foaming additive and other fillers toenhance the internal mechanical hysteresis of the rubber at normal roomtemperature, such as RTVlOS adhesive sealant silicone rubber sold byGeneral Electric Co. Note that there is a separate bonding and dampingelement 24 for each wall bonded to the housing front case 20, and thatboth of these bonding-damping elements are peripherally continuousaround the circumferential joints of the housing, including a leak-tightbond to and around electrical connector assembly 30. All joints of theear receiver housing rnust be gas-leak-tight, except of course soundport 21. The thickness of the damping element 24 should preferentiallybe no more than one-tenth of the axial plus radial length of one of thedamping elements 24. The length and thickness of the joint areproportioned to preserve the pressure vessel stiffness of the housing.

The rear walls 22 and 26 of this ear receiver housing are configured asa cup-within-a-cup with an intervening airspace. Each rear wall has afundamental mechanical resonant frequency differing from the other bypreferably at least 1/3 octave. The function of these separately damped,plural, and mutually detuned walls is the efective containment ofacoustic leakage which occurs at unavoidable Wall resonances. Damping atthe periphery of such cup shapes is effective particularly when theperipheral edges are thinned to a significantly narrower section asdiscussed above. The compound or 3-dimensional curvatures of the cupshapes are each chosen to achieve the pressure vessel radial stiffnessof at least 500,000 p.s,i, per inch for each cup. The material for therear wall cups, like housing front case 20, should have a specificacoustic impedance in excess of 7,000,000 kg. per secondXmeter2.Aluminum suitable for anodizing serves well for this purpose.

Because of the requirements for detuning, the thickness for the internalrear wall 26 usually but not necessarily exceeds the thickness of theouter Wall 22. The airspace between walls 22 and 26 serves to uncouplethe free mechanical vibrations of the said walls by a substantialfurther. reduce. through-owing energy. In the case of this invention,vibro-acoustic energy is mechanically .filtered by tuning relationshipsdrawn in FIG. 10. It is recognized that overtone resonances of cupshapes like bells are not in general integral multiples of thefundamental resonance. Because thel outer rear wall 22 is proportioneddifferently from the inner rear Wall 26, the overtones of thetwo Wallswill not in general be inthe same musical interval relationship to theirrespective fundamentals. The outer rear wall 22 features a narrowannular impression near to its outer diameter which serves to reducevisually the apparent size of the housing structure.

In the electrical connector assembly (FIG. 3) an insulating block 32 isconfigured to mate with another insulating block 34, and at the sametime to retain and position by mechanical recess means, the electricalspring contacts 36. The surface between the insulating blocks 32 and 34is made gas-leak-tight by means of a permanent rigid cement which alsoseals the hairline air passage around the Shanks of electrical springcontacts 36 where they protrude into the interior of the ear receiverhousing. The cemented assembly of insulation blocks 32 and 34 togetherwith electrical spring contacts 36 accepts by guide holes and springaction the prongs of the amplifier connection cord plug 38. Jumper leads(not shown) connect the transducer 28 to spring contacts 36.

The exterior surfaces of' insulating blocks 32 and 34 are made to locksecurely into the elements 20, 22, and 26 by means of damping-sealant24. Thus in this design air cannot leak from the housing through theconnector 30 or the damping seams 24 under small static pressures, sayunder tenvpounds per square inch.

In addition to the preferred arrangement of this invention as drawn inFIGS. 1 through 4, adaptation devices in accordance with the inventionmay improve existing styles of hearing aid ear receivers. FIGS. 5, 6,and 7 illustrate the ways in which the two major styles 52 and 70 ofhearing aid ear receiver are adapted in accordance with the invention.

The receiver S2 (FIGS. 5 and 6) is the standard and most common style ofear receiver; a plastic rear housing shell is joined, by rolling, to analuminum front wall. In FIG. 5, a stiff (as specified above) smallcup-like housing is attached to plastic-backed ear receiver 52 by meansof adhesive-sealant-damping material 54, thus adding a second rear wallto the housing of the ear receiver. The electrical plug 38 is coatedwith a release agent prior toinsertion and sealing into the assembly viaa port in the housing 50. The release agent allows removal of the plug38 for periodic cord replacement after sealing. After insertion of theplug 3S, the adhesive-sealant 56 shuts off leakage radiated from insidethe metal shell 50. Front wall damping 40, of mechanical dampingmaterial as described above encircles and seals the void between theearmold 51 and the stiff housing 50. A release agent between 58 and 51and between 50 and 51 allows removal of the earmold for periodicmaintenance. The plastic back of the ear receiver and housing 50 eachcan have a` different fundamental resonant frequency.

The device of FIG. 6 is similar to that of FIG. 5; but a largerover-housing-cap 60 is connected to small housing cap 50 by means ofadhesive-sealant-damping matei rial 62. Adhesive-sealant-dampingmaterial 68 encircles acoustical impedance mismatch. A vacuum would be lsuperior to air in acoustical impedance mismatch, but

and seals the void between the earmold 51 and the overhousing 60.Adhesive-sealant-damping material 64 seals off the leakage from withinoverhousing 60, completing the second air-tight housing wall. Theplastic back of the ear receiver, cap 50, and over-housing-cap 60 caneach have a different fundamental resonant frequency in this triple-wallhousing.

The ear receiver 70 (FIG. 7) is the type in which a thick aluminum backshell is joined to a thin steel front Wall -by a rolled ferrule rim 71.Leakage fr'om this type of ear receiver is primarily through the thinfront plate at its mechanical resonance, and secondarily throughhairline cracks under the ferrule 71. -In the arrangement of FIG. 7, themodification kit is applied to the stiffbacked type of ear receiver 70.Damping material 72 reduces resonance effects in the thin front plate ofthis ear receiver. Adhesive sealant 74 seals leakages through hairlinecracks at the rear of the ferrule 71, and adhesive sealant 76 attaches asmall housing cap 50 to the ear receiver 70 while sealing leakage aroundthe periphery of the joint with the earmold 51. If desired, theelectrical plug 38 in FIG. 7 may be caulked by material such as 56 inFIG. 5.

It is seen by FIGS. 5, 6 and 7 that it is possible to apply designcriteria A, B, C, D, and E above to ear receivers now being built andused throughout the world, by means of over-fitting housings likedesignated 50 ad 60 together with adhesive-sealant-damping materialssuch as room temperature vulcanizing silicone rubber, and releaseagents. It is moreover possible to supply housings 50 and 60 in colorsto match more closely the skin tone of the wearer, making the earreceiver less noticeable on the person.

Although not commonly thought of as such, a hearing aid operating closeto the point of howling feedback is an amplifying servo system withpositive feedback or regeneration. When viewed in this manner, furtherunderstanding of the invention is possible. Referring to the servosystem diagram of FIG. 8, environmental sound S enters the microphone(transmitter)-amplifer-transducer box G to create amplified sound A fromwhich some acoustic energy crosses the acoustic leakage pathfilter F tosustain regenerative acoustic energy R which reenters G. Notice that allsymbols in FIG. S are complex vector quantities, and that F particularlycontains a significantly time delay. Because of the geometric flexibiltyof feedback path F together with its time delay and standing waves, itis guaranteed that some positive inphase acoustic regeneration energy Rwill occur at some frequencies and negative out-of-phase energy willoccur simultanaeously at other frequencies. The combination of positiveand negative feedback operating simultaneously at different frequenciestends to destroy any linearity of the frequency amplitude profile of thehearing aid system. And of course any leakage 'R from an ear receiver isthe closest continuous sound source in a hearing aid system.

Output A of hearing aid systems can be be expressed in terms of othersystem elements as which by rearrangement:

=system amplitude gain behavior Notice that whenever the G/F complexratio approaches plus unity a volume expansion occurs at thosefrequencies preferentially selected by feedback path -fllter F. If Gequals or exceeds 1F at any frequency, the system amplification is onthe threshold of becoming unstable, contingent upon ythe vagaries ofshifting phase relationships in F. Of course during system instability,the output of the system runs away to its power capacity saturationlimits, and amplification by the system of other frequencies isdistorted by the saturating oscillation at feedback selectedfrequencies.

After a hearing aid system has begun howling, it is possible to changethe howl frequencies by changing the distance and/ or orientation of theear receiver and microphone. The howl frequencies usually jump frompitch to pitch rather than slide smoothly during this spacing change,confirming that feedback path F is not uniform with respect tofrequency. Acoustic standing waves and 10 tuned leakages 86 and 92 allcontribute to feedback path non-linearity with respect to frequency.

Just as the gain of a hearing aid system is increased at selectedfrequencies by the regeneration factor the time for the hearing aidsystem to reach this amplitude is incerased by the same factor. A fewcycles of the feedback frequency can be amplified by G withoutdistortion. But in a few milliseconds, when acoustic feedback AR arrivesat the hearing aid input, the emphasizing amplification or degenerationoccurs spontaneously. This feedback action then is a rapid-acting Volumeexpander for some specific frequencies passed by path F. Distorteddynamic ranges of amplified sounds, making of non-expanded or perhapscancelled frequencies, and extreme harmonic distortion andintermodulation distortions are only some of the system defects causedby acoustic feedback, regardless of the quality of the systemselectronics, even without system howling. The worst system defects arisebecause of the concurrence of the above volume expansion effects withthe high threshold of deafened sound perception to be explained morefully below. At this stage of the explanation it is fully understandablewhy many hearing aid wearers prefer to adjust their amplification volumecontrol to slightly below the incipient feedback level and so obtain abroader frequency range of response with less distortion at the cost ofoverall amplitude. Only the partially deaf can waste amplification G toobtain better quality hearing.

Some hearing aids have been constructed to function as volumecompressors. The object of the volume cor'npressor type of hearing aidbeing that full natural dynamic power (volume) ranges of sound can becompressed into the smaller dynamic hearing range between the deafenedperception threshold and the upper limits of sound pressure endurance(painfully loud sounds). But notice that the volume expansion effects ofacoustical feedback largely accounts for the disappointing performanceresults obtained from volume compressor types of hearing aids for thedeaf. The slower volume compression by electronics has been largelycancelled by the very fast acoustic regeneration effects by feedback.Those very persons who most need volume compression are those mostharmfully affected by feedback. Cruder Volume compression action bysaturation of ouptut circuitry with consequent :truncation of waveformis a common distorting characteristic of hearing aid amplifiers whichserves as a safety device against injury to an ear.

The above rationalization of hearing aid feedback yields furtherinsights. FIGS. 11 and 13 depice hearing aid system amplificationbehavior, given uniform input sound intensity (eg-white noise), as afunction of frequency and volume control setting respectively withoutand with acoustical feedback. Curve 101 shows the maximum amplificationobtainable within the upper limit imposed in FIGS. 13 and 14. The samelimit is used for system comparisons. FIG. 12 again plots curve 101 as aslice through the surface of FIG. 11. The sound perception threshold 102of a very deaf person is dotted across FIGS. 12 and 14. The area 104enclosed by curves 10-1 and 102 represents the deaf persons audiointelligence window with area measurable in decibel times octaveproduct.

As the gain of a hearing aid is advanced toward the incipientinstability or ringing condition, large peaks of time-delayedamplification arise on the frequency versus amplification profile. Curve106 on FIGS. 13 and 14 shows the maximum amplification obtainablewithout reaching the instability amplification level which is drawn asthe fiat upper plane truncating the amplification surface at all otherfrequencies. Of course, the curves of FIGS. 13 and 14 are simplified toshow only a single resonance frequency peak representations of realfeed- 'backs multiple peaks and valleys in hearing aid systems.

The area 108 enclosed by curves 102 and 106 in FIG. 14 represents thedeaf persons perception window as provided by an aid set at the ringingcondition; very little intelligence can be transmitted through such afrequency and volume range limited window. Of course as the deaf personshearing diminishes, curve 102 rises; and the persons hearing perceptions(through hearing aids necessarily operating near incipient howl) isincreasingly limited to the power peaks of volume-expanded, timedelayedsounds restricted moreover to the frequencies of incipient feedback.This selective emphasis of any one frequency of incipient feedback isthe same as losing all the other frequencies and destroying anyprescribed frequency/ amplitude profile.

If sound leakage in the feedback path F (elements 80, 82, 84, 86, 90,92, and 94) could be made uniform with respect to frequency, then lesselectrical gain G would be required for pure continuous tone perception;but the volume expansion effects of this acoustic regeneration woulddestroy the dynamic realism of the reproduced sound. Sharp percussivesounds would be distorted into g-runts at the perception power level ofthe very deaf people. Realizing the above, it is easy to understand whythe nearly totally deaf speak indistinctly in grunts (lacking clean cutconsonant sounds) yet in faithful reproduction of what they perceivethrough their bodyworn hearing aid equipment. The above is also acaution in interpreting pure continuous tone (or tones that are slowlyswept or warbled across a small frequency range centered on thefrequency of interest) audiometry with hearing aids as worn. To theextent of volume expansion, such audiometry probably indicates anoptimistic degree of hearing assistance. Widely spaced ping or dit 20millisecond or less tones are better.

Therefore all sound leakage must be minimized as those audio frequenciesbeing amplified. Acoustic amplication is uniquely distorted by acoustcfeedback prior to system howling. Insulation against feedback ismeaningfully measured at its most leaky frequency. Hearing aid maximumamplification can be no better or greater than its least insulationagainst feedback.

It should be understood that it is not desired to limit the invention tothe exact details of construction and configuration herein shown anddescribed because many modifications within the scope of the claims mayoccur to persons skilled in the art. Having now particularly describedand ascertained the nature of this invention, and in what manner thisinvention may be physically realized, I claim and desire to secure byLetters Patent:

1. An acoustic feedback resisting cap mechanism for hearing aid earreceiver housings comprising a cup shaped wall element with radialstiffness in excess of 500,000 p.s.i. per inch computed as a pressurevessel superimposed upon the housing of the hearing aid ear receiver,and means for providing a gas-leak-tight joint between the -Wall elementand the housing.

2. The invention as set forth in claim 1 above, wherein the ear receiverhousing front face abuts an earmold, and wherein in addition dampingmeans providing a seal is disposed between the housing front face andthe earmold.

3. An acoustic feedback resisting cap for commercially available hearingaid ear receiver housings comprising at least a pair of concentric cupshaped wall elements whose resonant frequencies differ and sealant meanscoupling the wall elements to a hearing aid ear receiver on the side ofthe ear receiver opposite the ear of the wearer.

4. An acoustic feedback resisting cap mechanism for commerciallyavailable hearing aid ear receiver housings, said housings having ameasurable resonance characteristie and an electrical plug connectionfor receiving an external plug, the mechanism comprising at least onecup shaped wall element with radial stiifness in excess of 500,000p.s.i. per inch computed as a pressure vessel superimposed upon andencompassing the housing at the side spaced furthest from the ear, anddefining an interior space having a substantially lower acousticimpedance than the housing and the wall element, the Wall elementincluding an aperture exposing the electrical plug connection, meansdisposed on the wall element providing acoustic damping thereof, andmeans disposed on the periphery of the housing and between the housingand vthe wall element for providing a gas-leak-tight joint therebetweenabout the housing and the electrical plug connection, means disposed onthe wall element providing acoustic damping thereof, and means disposedon the periphery of the housing and between the housing and the wallelement for providing a gas-leak-tight joint therebetween about thehousing and the electrical plug connection.

5. A hearing aid ear receiver housing comprising a wall element disposedon the opposite side of the transducer within the receiver from the earwith radial stiffness in excess of 500,000 p.s.i. per inch computed as apressure vessel, the wall element partially encompassing the earreceiver, and means disposed about the inner periphery of the wallelement providing gas-leak-tight coupling to the ear receiver.

6. A housing for a hearing aid ear receiver comprising in combinationgenerally hemispherical cap means disposed about and spaced apart fromthe transducer, the cap means having a wall radial stiffness in excessof 500,000 p.s.i. per inch computed as a pressure vessel, and sealingmeans disposed about the cap means and coupling the cap means to thetransducer about its periphery and providing a substantiallygas-leak-tight joint.

7. A hearing aid ea-r receiver housing comprising in combination atleast a pair of substantially concentric wall elements whose reasonantfrequencies diifer and including gas-leak-tight joint means coupling thewalls elements to the housing.

8. An ear receiver housing having in combination wall means disposedabout the transducer of the ear receiver and electrical connectorassembly therefor with radial stiffness in excess of 500,000 p.s.i. per'inch computed as a pressure vessel, housing joint means resistant togas pressure leakage from the interior of aid housing to its exteriorand disposed about the Wall means, and stiff and gas leakage resistantmeans sealing the wall means about the electrical connection assembly.

9. The invention as set forth in claim 8 above, wherein the ear receiveris seated in an earmold, and wherein a front face labyrinth seal isprovided between the earmold and the ear receiver portions.

10. In a hearing aid ear receiver, a housing to contain acousticleakage, the receiver including a transducer and normally attached to anassociated earmold, and the housing comprising: a pair of rear wallsdisposed on the opposite side of the transducer from the earmold, atlleast one of the walls being of a radial stiffness in excess ofapproximately 500,000 p.s.i. per inch radial bulge computed as apressure vessel and comprising material having an acoustic impedance inexcess of approximately 7,000,- 000 kg./sec.m.2, the walls having otherthan the same resonance frequency.

11. In a hearing aid receiver, a housing mechanism to contain acousticleakage in undesired directions from the receiver, the receiverincluding a transducer and normally attached to an associated earmoid,the housing mechanism comprising wall means disposed to at leastpartially encompass the receiver and to form plural walls relative tothe transducer, at least one of the plural walls being generallyhemispherical and having a radial stiffness in excess of approximately500,000 p.s.i. per inch radial bulge when computed by the pressurevessel stilfness criterion, said wall also being of a material having anacoustic impedance specificati-on in excess of approximately 7,000,000kg./ sec.m.2, and being gas-leak-tight, said walls also havingIdifferent fundamental mechanical resonant frequencies; the acousticimpedance of said wall means being substantially higher than the spacedapart portions therebetween;

and acoustic damping means disposed individually on at least one of saidwalls.

12. The invention as set forth in claim 11 above, wherein the wallcloser to the earmold includes means sealing the wall means to theearmold at the periphery of the wall, thus to contain acoustic leakagefrom front base vibrations and leakage from the sound port joint of theearmold.

13. For hearing aid ear receivers of the type including a circularreceiver base having a transducer mounted n one side and iitting into anearmold on the other, an annulus on the receiver base fitting into amatching groove in the earmold, the receiver including a cover capjoined to the receiver base on the side on which the transducer ismounted, and further including a connector socket for receiving Contactpins of an external connector plug, the combination comprising anexternal arcuate cap member having a periphery disposed about theperiphery of the receiver base member, at least partially encompassingthe cover cap, and spaced apart from the cover cap, the cap memberhaving a radial stiiness in excess of approximately 500,000 p.s.i. perinch radial bulge computed as a pressure vessel, and both the cover capand the cap member being of a material having an acoustic impedancespecication in excess of approximately 7,000,000 kg./sec.m.2, andincluding an aperture for receiving the connector plug, the cap memberfurther tapering at its periphery to provide a mechanical transformeraction, the resonance frequency of the cap member differing by in excessof 1/3 octave from that of the cover cap; first sealant means providinga gas-leak-tight joint between the cap member and the receiver baseabout the periphery thereof; second sealant means disposed between thereceiver base and the cap member and providing a gas-leak-tight jointabout the connector socket; third sealant means providing agasleak-tight joint between the earmold and the receiver base andinterposed within the groove between the earmold and the annulus of thereceiver base; and acoustic damping means disposed between the broadfaces of the receiver Ibase and the earmold.

14. For hearing aid ear 'receiver type including an earmold and partmetal part plastic cover combination, a transducer being disposed withinthe part metal, part plastic cover, and the plastic rear cover alsoincluding an electrical socket connection lfor receiving an externalplug, the combination comprising: an external metallic cap member ofgenerally hemispherical form congured to receive the plastic cover withan intervening air space generally therebetween, the cap member having aradial stiffness in excess of approximately 500,000 p.s.i. per inchradial bulge computed as a pressure vessel, and the material having anacoustic impedance in excess of approximately 7,000,000 kg./sec.m.2, andthe cap member material and attachment means being gas-leak-tight;acoustic damping means joining and sealing the opposing faces of themetal front base cover and the earmold to contain acoustic leakage fromfront base vibrations and leakage from the sound port of the receiver;lirst sealant means disposed between the cover and the cap member aboutthe periphery thereof and providing a gas-leak-tight joint; secondsealant means disposed between the cap member and the earmold about theperiphery thereof and providing a gas-leak-tight joint; and thirdsealant means disposed about the electrical socket connection of theplastic cover and coupling the metallic cap member to the plastic coverand providing a gas-leak-tight joint about the electrical socketconnection, all of said sealant means being readily separable formaintenance without destruction of any of the principal members of thehearing aid receiver or the metallic member.

15. A hearing aid ear receiver including 1in combination an earmoldhaving a sound port; a hearing aid ear receiver member mechanicallycoupled to and disposed adjacent to the earmold and including atransducer within a metalplastic cover, the rear portion being plasticand also including an electrical connector socket for receiving anexternal connector plug; acoustic damping means disposed between theearmold and the metal front cover; a rst cap member generallyhemispherical in form encompassing the plastic cover on the sideopposite from the earmold and providing an air space therebetween, therst cap member having a radial stiffness in excess of 500,000 p.s.i. perinch radial bulge computed as a pressure vessel and being made of amaterial having in excess of approximately 7,000,000 kg./sec.m.2; rstsealant means being disposed between the plastic cover and the iirst capmember and providing a gas-leak-tight joint; a second cap member ofgenerally hemispherical form disposed about the rst cap member and thesecond cap member and providing a gas-leak-tight joint about theperiphery thereof on the side adjacent the earmold; third sealant meansdisposed between the second cap member and the earmold albout theperiphery thereof and providing a gasleak-tight joint; the cap membersalso including apertures for receiving on the plastic cover within theapertures defined by the rst and second cap members in the vicinity ofthe connector plug entry for providing a gas-leak-tight joint.

16. For a hearing aid receiver of the type including a transducer withina metal rear housing shell having a rolled ferrule rim, and an earmold,the combination comprising an additional generally hemispherical capmember disposed about the metal rear housing shell and providing anintervening airspace therebetween, the cap member made of material withan acoustic impedance in excess of 7,000,000 kg./sec.m.2, acousticdamping means disposed between opposing faces of the metal cap andearmold; rst sealant means disposed between the metal cap and the capmember at the peripheries thereof; and second sealant means disposedabout the joinder line between the ferrule rim and the metal cap, eachsealant means providing a gas-leak-tight joint.

References Cited UNITED STATES PATENTS 2,344,023 3/ 1944 Carlisle179-107 2,863,005 12/1958 Knauert 179-107 2,950,357 8/1960 Mitchell 1179-107 KATHLEEN H. CLAFFY, Primary EXamineI' A. A. MCGILL, AssistantExaminer UNITED STATES PATENT OFFICE CERTIFICATE OF CORRECTION Patent No3 ,457 ,375 July 22 1969 John W. Haggerty lt is certified that errorappears in the above identified patent and that said Letters Patent arehereby corrected as shown below:

Column 2, lines Z2 and 23, "incresingly" should read increasingly Column3, line 42, "exposed" should read exploded Column 4, line l5,"accordane" should read accordance line 52, "stiffness" should readstiffnesses Column 9, line 38, after "energyH insert R line 39,"simultanaeously" should read simultaneously Column l0, lirI 8,"incerased" should read increased line l5, "making" shoul read maskingline 5l, "depce" should read depict lir 73, after "simplified insert fColumn ll, line 35, "as" should read at Column l2, line 4l, "aid" shouldread said (SEAL) Attest:

Edward M.F1errher,1r. WILLIAM E. SCHUYLER, JR.

Attesting Officer Commissioner of Patents Signed and sealed this 31stday of March 1970.

